Gulf War Veterans Illness (GWVI) is a constellation of symptoms reported by Gulf War Veterans shortly after their return from deployment in 1991. Clinical diagnostic criteria for GWVI are based on chronic multisystem illness (CMI) criteria, which are based on statistical symptom cluster analysis resulting in three categories: fatigue, mood/ cognition, and musculoskeletal symptoms. Currently, approximately 40% of Gulf War Veterans (over 1/4 million Veterans) have GWVI by these criteria. The pathophysiological mechanisms underlying GWVI are not understood, and insights into the mechanisms of GWVI could lead to novel concepts, methodologies for study, and treatment interventions. Our Pilot study will assess three facets of vascular and skeletal muscle function in 25 cases with GWVI and 25 Veteran controls without GWVI. The purpose of the proposed three-year pilot study is to determine whether lower extremity (leg) endothelial function, exercise functions, and skeletal muscle mitochondrial gene regulation are different among Veterans with GWVI compared to Veterans' without this illness. Endothelial dysfunction, skeletal muscle functional impairment, and dysregulation of mitochondrial genes are plausible mechanisms for GWVI as exposure to anticholinesterase inhibitors during the Gulf War may affect these functions to cause symptoms of fatigue and other musculoskeletal symptoms. We will recruit subjects from a well characterized cohort of Gulf War Veterans (the Fort Devens Cohort). Approximately 60% of Gulf War Veterans in this cohort meet clear CMI criteria for GWVI. Symptoms and exposure to pesticides, pyridostigmine bromide, and low level sarin exposure are well documented in these cohorts. Cases and controls will have femoral artery microvascular endothelial function assessed invasively in the cardiac catheterization laboratory using Doppler flow wire and intravascular ultrasound. We will use this technique to measure flow and artery responses to intra-arterial infusions of the endothelium-dependent vasodilator acetylcholine 10-6M, the microvascular endothelium-independent dilator adenosine, and the large artery vasodilator nitroglycerin. The PI has a long track record in this type of research. After 1 week, subjects will have an extensive skeletal muscle functional assessment using cardiopulmonary exercise testing to measure anaerobic threshold, objective strength and fatigue assessments, and the 6-minute walk test. One week after this study, subjects will have a skeletal muscle biopsy from the thigh. This will be analyzed for muscle fiber type, and RT-PCR to assess nuclear and mitochondrial genes responsible for regulating mitochondrial respiratory function. The overall aim is to assess differences in pathophysiological and muscle mechanisms, which will allow us to design a more definitive MERIT Review proposal to assess differences in these functions and test potential treatments targeting these mechanisms. This Pilot study may also provide insights into other chronic illness characterized by fatigue and other musculoskeletal symptoms, such as peripheral vascular disease and congestive heart failure.